I have Roth conversions to do over the next several years that will cause us to 'break' the MFJ $170K IRMAA amount in at least one year. I have always presumed that for the years after that, when we are again below the $170K level, the IRMAA would be reset. I have not, however, been able to find anything that says that is the way it works, either here (on Bogleheads) or on the SS/Medicare websites.

(i) In general.—In applying this subsection for an individual’s premiums in a month in a year, subject to clause (ii) and subparagraph (C), the individual’s modified adjusted gross income shall be such income determined for the individual’s last taxable year beginning in the second calendar year preceding the year involved.

(i) In general.—In applying this subsection for an individual’s premiums in a month in a year, subject to clause (ii) and subparagraph (C), the individual’s modified adjusted gross income shall be such income determined for the individual’s last taxable year beginning in the second calendar year preceding the year involved.

Maybe.
The IRMAA tiers are supposed to resume cost of living adjustments starting in 2020.
I don't know what that means yet as regards one's MAGI for 2018 and 2019...

Yes, you are correct, of course. I haven't heard how that will be implemented either. Since Medicare MAGI's were frozen for 10 years (ending in 2020), maybe it would be enlightening to know what happened before 2010?

As I read
Sec. 1839(i)(5), the 2020 IRMAA thresholds are scheduled to be adjusted by the increase, if any, in the CPI from August 2018 to August 2019. The thresholds for 2020 will be published in September 2019. But by then it will be too late to change your 2018 MAGI.

The real risk is underestimating the 2020 MAGI value and then blowing through it.

If you just use the unadjusted 2019 value ($170,000) you would probably only be leaving a few $K of potentially unused limit value on the table. Say inflation for September 2018 through August 2019 is actually 3.0%. The 2020 MAGI would then be $175,100, so planning on using, say, $167,000 or so is probably close enough.